The present invention generally relates to marine geophysical surveying and, more particularly, in one or more embodiments, to methods and systems for using tension feedback from steerable deflectors to adjust vessel turn time.
Marine geophysical surveying systems such as seismic survey systems and electromagnetic survey systems may be used to acquire geophysical data from formations disposed below the bottom of a body of water, such as a lake or ocean. Such geophysical data may indicate geophysical structures and/or identify formations likely to contain useful materials, such as water, oil, or natural gas. Marine seismic surveying systems, for example, typically may include a seismic survey vessel having onboard navigation, seismic energy source control, and geophysical data recording equipment. The seismic survey vessel may be configured to tow a sensor streamer or, more typically, a plurality of laterally spaced apart sensor streamers through the water at one or more selected depths. One or more deflectors may be positioned outside of the sensor streamers to generate lateral thrust for maintaining a desired lateral spacing of the sensor streamers. At selected times, the seismic energy source control equipment may cause one or more seismic energy sources (which may be towed in the water by the seismic vessel or by another vessel) to actuate. Signals generated by various sensors on the one or more streamers in response to detected seismic energy may be ultimately conducted to the recording equipment. A record can be made in the recording system of the signals generated by each sensor (or groups of such sensors). The recorded signals may be interpreted to infer the structure and composition of the formations below the bottom of the body of water. Corresponding components (transmitters and receivers) for inducing electromagnetic fields and detecting electromagnetic phenomena originating in the subsurface in response to such imparted fields may be used in marine electromagnetic geophysical survey systems.
While the sensor streamers and other equipment are deployed behind the survey vessel, the survey vessel may need to execute a turn. For example, one type of survey may comprise a grid of straight lines in opposing directions wherein the survey vessel is actively generating geophysical data while moving in the straight lines. Accordingly, a 180-degree turn may be executed so that the survey vessel can be positioned over the next line to be shot in the opposite direction. Another type of survey may execute continuous turns in a circular or spiral pattern. Due to the number and length of the sensor streamers towed from the survey vessel, however, turning may be a complex procedure. In some instances, for example, when the survey plan does not generate geophysical data during turns, it may be desired to reduce the turn time for the survey vessel.
A number of different techniques may be utilized to reduce the turn time. For instance, one technique may include keeping the turn radius constant and increasing speed of the survey vessel. Another technique may include reducing the turn radius and thus the distance traveled while keeping the speed constant. Yet another technique may include a combination of a reduction in turn radius and increased vessel speed. In all of these techniques, a limitation can be the tension on the deflector. As previously mentioned, deflectors may be positioned outside the sensors streamers to generate lateral force for maintaining a desired lateral spacing of the sensor streamers. During a turn, the outer deflector should experience an increase in tension while the inner deflector should experience a corresponding decrease in tension. However, to ensure stable deflector operation, the tension on the outer deflector should not exceed a maximum value, and the tension on the inner deflector should be kept above a minimum value. There exists a need to adjust turn time while maintain stable functioning of the deflectors.